I have completely remade the video from the project file and am now uploading that.
It seemed to play through all right here, so I am reasonably confident that there is no fault in the source file.
Whether it'll get up the line successfully, a day or so hence, I shall not know until the end of that 28-hour period;
In the meantime, I have another GigaByte or so, in 51 video clips from today's Dickens Festival, to start cataloguing in preparation for editing all of that. I shall probably add something like as much source material again tomorrow, on the festival's final day.